1. Field of the Invention
The present invention relates generally to manufacturing processes requiring lithography and, more particularly, to monitoring of lithographic and etch process conditions used in microelectronics manufacturing which is particularly useful for monitoring pattern features with dimensions on the order of less than 0.5 micron.
2. Description of Related Art
Control of a lithographic imaging process requires the optimization of exposure and focus conditions in lithographic processing of product substrates or wafers. Likewise, it is also important to optimize etching and other parameters on product wafers. Generally, because of the variations in exposure and focus, patterns developed by lithographic processes must be continually monitored or measured to determine if the dimensions of the patterns are within acceptable range. The importance of such monitoring increases considerably as the resolution limit, which is usually defined as minimum features size resolvable, of the lithographic process is approached. The patterns being developed in semiconductor technology are generally in the shape of lines both straight and with bends, having a length dimension equal to and multiple times the width dimension. The width dimension, which by definition is the smaller dimension, is of the order of 0.1 micron to greater than 1 micron in the current leading semiconductor technology. Because the width dimension is the minimum dimension of the patterns, it is the width dimension that challenges the resolution limits of the lithographic process. In this regard, because width is the minimum and most challenging dimension to develop, it is the width dimension that is conventionally monitored to assess performance of the lithographic process. The term “bias” is used to describe the change in a dimension of a feature from its nominal value. Usually the bias of interest is the change in the smallest of the dimensions of a given feature. Further, the term “bias” is invariably used in conjunction with a process such as resist imaging, etching, developing etc. and described by terms such as image bias, etch bias, print bias etc.
More recent lithographic monitoring improvements have been in optical metrology which rely on human or machine-read visual measurement of targets which employ arrays of elements having line widths and spacing below the wavelength of the light used to make the measurements. Improvements in monitoring bias in lithographic and etch processes used in microelectronics manufacturing have been disclosed in U.S. Pat. Nos. 5,712,707; 5,731,877; 5,757,507; 5,805,290; 5,953,128; 5,965,309; 5,976,740; 6,004,706; 6,027,842; 6,128,089 and 6,130,750, the disclosures of which are hereby incorporated by reference. The target and measurement methods of these patents rely on the increased sensitivity to process variation provided by image shortening. Some of these types of targets use image shortening effects to make the visual measurements of even though the individual array elements are not resolvable. Examples of such targets are disclosed in the aforementioned U.S. patents. Such targets permit visual monitoring of pattern features of arbitrary shape with dimensions on the order of less than 0.5 micron, and which is inexpensive to implement, fast in operation and simple to automate. These determine bias to enable in-line lithography/etch control using optical metrology, wherein SEM and/or AFM metrology is required only for calibration purposes.
The lithographic process window is the dose and focus space over which a set of features can be printed (i.e., exposed and etched) within allowable tolerances on a given mask level. Consequently, accurate and efficient means of measuring effective dose and focus simultaneously, referred to as process window metrology (PWM), are essential to lithography characterization and control. Prior art has established PWM methods that involve the use of dual-tone targets, such as those disclosed in U.S. Pat. No. 5,976,740. A drawback of these methods is that some product levels either do not allow the printing of dual-tone targets due to ground rule violations, or require exposure in a regime where the dual-tone targets do not provide adequate response. Starikov, in “Exposure Monitor Structure”, Integrated Circuit Metrology, Inspection and Process Control IV, SPIE Vol. 1261, pp. 315-324, (1990), proposed an exposure monitor structure that is sensitive to dose, but not focus, using a fine structure that is not resolved by the optical lithography tool used for printing and which functions as a transmission wedge. Because the Starikov structure is only sensitive to dose, it is not able to determine the effects of focus.
In addition to the aforementioned problems in visual monitoring, the prior art also does not provide for electrically testable monitors to separate the effects of dose and focus.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide both a target mask and a target which may be used to evaluate and separate lithographic dose and focus variations.
It is another object of the invention to provide electrically testable structures to separate the effects of dose and focus in lithographic processing.
It is yet another object of the present invention to provide a method of separating the effects of dose and focus in lithographic processing in a target or monitor which can be formed on a single lithographic layer on a wafer substrate.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.